1. Field of the Invention
The invention relates to a method of laminating at least two foils such as plastic foils, metal foils and ribbons, paper, or fabric, the foils being treated with a low energy gas discharge in vacuo. Moreover, the invention relates to a device for the application of this method.
The field of the invention is laminating two or more foils without using adhesives.
2. Description of the Prior Art
Compound materials consisting of several layers, especially compound foils, are more and more used. Special properties different from those of mono foils can be achieved by selection of suitable materials. A main field of application of compound materials is packaging technology, and flexible laminated foils playing the most important role in this connection. Most foil combinations are manufactured by laminating previously manufactured mono foils.
Today we know numerous methods of bonding foils together. In particular, the following adhesives are used: waxes, thermoplastic adhesives, polymers, glues, dispersions, adhesive varnishes, and reactive adhesives. The basic materials for the production of all these laminating adhesives, except some glues, are highly refined mineral oil derivates which are produced with a high expenditure of energy.
Waxes, thermoplastic adhesives, and polymers are applied molten and undiluted, the average quantity being 15 to 20 g/m.sup.2. The adherency of the compounds is sensitive to water and relatively low.
Using glues and dispersions, the applied quantity of solids is about 2.5 g/m.sup.2, on an average. These adhesives must be applied diluted, the mainly used thinner being water. During the laminating process this water must be evaporated by application of process heat, thus heating the entire compound. Thereafter the compound must quickly be cooled down. It is true that with this method the consumption of adhesives is lower than with the methods mentioned before, but the toal amount of required process energy is considerably higher. The adhesive power of the compound is high and more heat-resistant. The method cannot be applied unless at least one of the materials to be laminated is highly permeable to steam.
So far the application of adhesive varnishes results in the highest adhesive power; it is possible to use adhesive varnishes with nearly all compound materials. The mainly used adhesives are heat-reactive and dissolved in organic solvents. The average consumption of adhesives is 12 g/m.sup.2, the quantity of solids being 4 g/m.sup.2 and the quantity of solvents being 8 g/m.sup.2. These solvents must be evaporated by application of process heat. In addition, they must be removed from the waste air for reasons of air pollution control. Regarding the high energy consumption, the problems of air pollution, and the resulting higher costs of investment and operation, this method is very disadvantageous.
As for the adherency properties which can be achieved the reactive adhesives correspond to the adhesive varnishes, approximately. At present, however, the universal application of such reactive adhesives is not yet possible as the methods are still in the phase of development and introduction. The reactive adhesives are applied without using solvents and less than 2 g/m.sup.2 is needed. Hereby the air pollution problems are solved and the energy consumption is largely reduced.
The treatment of foils surfaces with a corona discharge, i.e., a high frequency gas discharge under atmospheric pressure, is known from U.S. Pat. No. 3,823,061. After the treatment the surfaces are bonded together. The adhesive power of such a compound foil is, however, not completely satisfying and only a few sorts of foils can be used with this method.
The corona treatment of metallic foils such as aluminum foils is known from DAS No. 1,222,241; DAS No. 1,236,904; and DAS No. 1,241,682. The metallic surfaces treated in order to improve the adherency are subsequently either varnished, or laminated by means of adhesives, or extrusion-coated with synthetic materials such as polyethylene.
The corona treatment of non-conductive synthetic foils for the same purpose is also well known. A survey of the potential fields of application of the electric treatment of materials and surfaces is given in the article "Prospects for industrial applications of electrical discharge", Chem. Tech., April 1971, p. 232-237. This article gives, among others, a description of the above mentioned discharge treatment of plastic foils in order to improve the adherency while the foils are extrusion-coated with thin synthetic layers of polyethylene, e.g. This extrusion coating method with corona treatment already does not use any additional adhesives nor coupling agents. On the contrary, the molten synthetic layer acts as if it were an adhesive itself.
A description is also given of devices for the adhesive-free lamination after preceding electrical or corona treatment, cf e.g., U.S. Pat. Nos. 3,329,549, 3,360,412 and 3,823,061. However, these devices are generally only suitable for the treatment of a specific type of material or product and do not have completely satisfactory joining strenths.
Recently, the patent literature has referred to another method for the surface treatment of materials, the so-called "plasma treatment". This is a low energy gas discharge in vacuo, which is described inter alia in U.S. Pat. Nos. 3,477,902; 3,647,592; and 3,959,567. DOS No. 3,124,280 describes a continuous installation for the treatment of foil material. In addition, U.S. Pat. No. 3,959,567 disclosed a device for producing laminates, following a preceding plasma treatment. Compared with corona treatment and other electrical and in particular electrostatic treatments, the joining strength of the composite materials produced by lamination after plasma treatment is clearly improved, as is proved by the values in the table at the end of the description.
In summarizing, it can therefore be assumed that it is known in the corona treatment field to laminate foils, i.e., to join them together. This treatment takes place in a common chamber, cf U.S. Pat. No. 3,329,549.
It is also known to treat foils with a low energy gas discharge in vacuo, i.e., to perform a so-called plasma treatment. The known device treats a foils, without there being any direct connection to another foil, cf U.S. Pat. No. 3,477,902.